Wireless sensor nodes (WSNs) in Internet of Things (IoT) applications require radio to be both low-power and small-area, in order to extend the battery life time and reduce the hardware cost. Meanwhile, in some IoT applications, the slave nodes require swift response upon receiving the signals from the master node. Therefore latency is critical, meaning that the receivers of the remote sensors have to be always on or frequently on in order to meet the latency requirement.
Monitoring channels continuously by a main radio can be quite power hungry for the system, with regard to the small battery capacity for these applications. Wakeup receiver (WuRx) based approaches are widely adopted to solve such issues.
WuRx is monitoring the channel continuously instead of the main radio, and therefore has to be low-power. To minimize the power consumption of WuRx, complex modulations (e.g., frequency-shift keying (FSK), phase-shift keying (PSK)) are often avoided due to necessity of a Local Oscillator (LO) generation, which either is power-hungry or needs calibration. Instead, simple On-off keying (OOK) modulation is typically adopted for the proprietary WuRx, enabling the deployment of a low-power energy-detection based receiver architecture. However, OOK modulation cannot be provided by most of the standard compliant radios. In addition, all of the aforementioned approaches require a customized wakeup transmitter, increasing system overhead. On the other hand, constant envelope based modulation schemes, such as Gaussian frequency-shift keying (GFSK) and Half-Sine offset quadrature phase-shift keying (HS-OQPSK), are adopted by Bluetooth low energy (BLE) and IEEE802.15.4 standards, which sheds light on a new wakeup radio design approach.
Some conventional systems include a WuRx, which directly operates with BLE advertising packets, but such systems suffer from large latency (˜seconds), which can be a problem for latency-critical applications.